We probably have been guilty of short-changing geothermal energy as a possible solution to the world's energy problems, in our discussions at this website. Geo-energy.org reports that installed capacity worldwide is already 13.3 GW. For whatever it's worth, the UN has pledged five-fold growth from that base by 2030.
There are huge geothermal energy resources, but also huge challenges in designing and building equipment that can withstand the enormous temperatures and pressures deep under the earth.
Graphene is a very exciting new technology, and the ability to make stranded graphene cable (as shown in the above video) is a huge innovation -- if they can really do it. Graphene has huge thermal conductivity & tensile strength.
It's certainly possible that "Billions in Change" are correct, that graphene could be a key element in expanding the use of geothermal energy.